Process for preparing submicron/nanosize ceramic powders from precursors incorporated within a polymeric foam

ABSTRACT

A process for preparing uniform, agglomerate free, submicron/nanosize ceramic powders from a polymeric foam comprising metal cations homogeneously incorporated within a foam cell structure of the polymeric foam. The polymeric foam is heated to remove any solvent, and calcined at a temperature of about 400° C. to about 1200° C. for about 1/2 to about 8 hours to produce the desired ceramic or metal powder.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process for preparing uniform,agglomerate-free, submicron/nanosize ceramic powders for use as startingpowders for high technology ceramics. In particular, this inventionrelates to a process for preparing uniform, agglomerate-free,submicron/nanosize ceramic powders from a polymeric precursor solutionincorporated into a polymeric foam.

2. Description of the Prior Art

The usefulness of many high technology ceramics depends upon thecharacteristics of the ceramic powders used as starting powders whichare ultimately sintered to produce a dense ceramic. Ideally, to achievedesirable characteristics in the finished products, a ceramic powdershould consist of dense particles with a narrow size distribution in thesubmicron range. In addition, to avoid pores larger than the primaryparticle size, the particles should be discreet, rather than attachedtogether in agglomerated clusters. Agglomerated clusters often producelow-density green ceramics and leave numerous large pores aftersintering. Finally, it is important that the ceramic powder be free ofcontaminants to insure purity of the resulting high technology ceramic.

One known method of preparing powder formulations used in hightechnology ceramics involves the calcination of a mechanically groundmixture of metal oxides and carbonates in definite proportions. Themilling and grinding introduces contaminants from abrasive materialswhich have a detrimental effect o the desirable properties and introducea variance into each batch of powder prepared. In addition, themechanically ground mixture requires prolonged calcination at hightemperatures which promotes crystallite coarsening, an undesirableconsequence in the fabrication of dense fine grain ceramics. U.S. Pat.No. 3,330,697 teaches a process for preparing lead and alkaline earthtitanates and niobates from resin intermediates made fromalpha-hydroxycarboxylic acids, such as citric acid, in which a hydratedoxide or alkoxide and an alpha-hydroxycarboxylate of titanium, niobium,and zirconium is mixed with citric acid in a polyhydroxy alcohol whichis liquid below about 100° C., dissolving therein at least one basic onemetal compound from the group of oxide, hydroxide, carbonate andalkoxide of lead and the alkaline earth metals, and calcining thecomposition to remove the organic constituents. The resulting productincludes agglomerated particles which require grinding aftercalcination. In addition, particle size is very difficult to control bythis method.

Similarly, Chick, L. A. et al., "Synthesis of Air-Sinterable LanthanumChromite Powders", Proceedings of the First International Symposium onSolid Oxide Fuel Cells, vol. 89-11, pgs. 171-187, teaches a process forsynthesizing lanthanum chromites in which metal nitrates and glycine orsome other low molecular weight amino acid are dissolved in water andthe resulting solution is boiled down until it thickens and ignites,producing ash that contains the oxide product. Thereafter, the oxideproduct is calcined, sonicated and dry pressed. This process tooproduces agglomerates which require grinding after calcination, therebyintroducing contaminants into the ceramic powder. In addition, particlesize is very difficult to control.

Numerous methods for preparing porous ceramic materials havingparticular physical and chemical properties are disclosed by the priorart. One such approach is disclosed by U.S. Pat. No. 2,918,392, U.S.Pat. No. 4,004,933, U.S. Pat. No. 3,907,579, U.S. Pat. No. 3,833,386,and U.S. Pat. No. 4,559,244 in which a foam or porous solid body isimpregnated with a material for deposit on the surfaces of the foam orporous body and subsequently treated, for example sintered, to produceporous ceramic or ceramic coated materials. U.S. Pat. No. 3,649,354teaches a method for producing electrically operated devices in which aliquid electrically insulating filler material, such as polyurethane, isapplied to a layer of electrically active grains and allowed tocontract, thus exposing the peaks of the grains, after which it isallowed to harden. U.S. Pat. No. 4,572,843 teaches a method forproducing a capacitor in which an insulating composition, such as anorganic polymeric compound containing a metal powder or anorganometallic compound as a metal source is formed on a dielectriclayer formed on an electrode, the insulating composition being heated toform a second conductive electrode.

With a somewhat different approach, U.S. Pat. No. 3,497,455 teaches amethod for producing foam metallic oxides in which an aqueous solutionof a metal salt (nitrate) is mixed with a frothing agent to form anoncollapsing foam, and subsequently heated up to about 3000° F. to formthe porous product.

Numerous other methods for preparing ceramic structures from ceramicpowders are also disclosed by the prior art. U.S. Pat. No. 4,957,673teaches a method for producing unitary layered ceramic structures havingcosintered layers for use in fuel cells, such as tapes having a centerlayer of yttria, stabilized zirconia sandwiched between outer layers ofstrontium doped lanthanum manganite. U.S. Pat. No. 2,108,995 teaches ananode of film forming material and a cooperating cathode spaced by asheet of a flexible nonfibrous albuminous sheet material which has beenimpregnated and made electrically conductive by the addition of aconductive electrolyte, such as ethylene glycol and citric acid.Similarly, U.S. Pat. No. 2,158,981 teaches an electrolytic condenserhaving a highly viscous or pasty electrolyte where the electrolyte is,for example, citric acid and ethylene glycol.

U.S. Pat. No. 3,180,741 teaches a method for producing solid productsfrom liquid polymers with polyvalent metallic salts using a mono- orpolycarboxylic acid. U.S. Pat. No. 3,386,856 teaches a method formanufacturing a device consisting mainly of oxidic dielectric materialin which the device is provided with electrodes and at least one of thesurfaces of the device on which the electrodes are provided issuperficially oxidized until an insulating junction layer is formed.

U.S. Pat. No. 3,427,195 teaches a process for producing an electrolyticcondenser in which a metal foil is coated with a liquid film of watersoluble nitrates or oxalates and finally suspended particles of a waterinsoluble refractory compound and heated to produce an electricallyinsulating refractory oxide which, together with a refractory compound,forms the separator coating on the metal foil.

Various methods for preparing dielectric ceramic powders are also taughtby the prior art including U.S. Pat. No. 3,647,364 which teaches aprocess for preparing high purity, submicron, dielectric ceramic powdersusing alcoholates; U.S. Pat. No. 3,965,046 which teaches a process formaking metal bearing powders from organometallic salt seeds; U.S. Pat.No. 4,004,917 which teaches a process for producing acicular metallicpowders from organometallic salts by precipitation or growth of the saltin the presence of complexing agent; similarly, U.S. Pat. No. 4,146,504which teaches a process for producing structures formed from powders ofhigh porosity made using organometallic salts and glycol; U.S. Pat. No.4,751,070 which teaches a method for synthesizing submicron particles ofceramic or metallic materials at very low temperatures in which anitrate source is combined with an inorganic reducing fuel to provide achemical precursor for the particular ceramic or metallic material,which precursor is exothermically decomposed in a controlled atmosphereat temperature of about 200° C. below the endothermic decompositiontemperature of the nitrate source; U.S. Pat. No. 4,757,037 in which asuspension formed from a mixture of a solution of titania containingelementary crystallites of titanium oxide and a solution of neodymiumnitrate or a solution of barium and neodymium nitrates is dried toobtain a dried product and calcined at a temperature of 800° to 1300 °C. to obtain an ultrafine dielectric powder; U.S. Pat. No. 4,800,051which teaches a method for ceramic fabrication involving hydrolyzing asuitable metal alkoxide to form a slurry, drying the metal oxide powderin the slurry, granulating and calcining the metal oxide powder,ballmilling the calcined metal oxide powder as a slurry to maximizepowder dispersion in the solution, compacting the dispersed powder fromthe ballmilled slurry into a powder compact, drying the powder compactand sintering the powder compact at a suitable relatively low sinteringtemperature; U.S. Pat. No. 4,845,056 in which a solution of ceramicoxides or hydrous oxides is continuously pressurized and heated to inexcess of the critical temperature and pressure of the solution solvent,transforming the solvent to a gas and subsequently separating it fromthe fine particulate ceramic oxide powder; and U.S. Pat. No. 4,141,763in which a stream of an aqueous solution of metal salt and a solutioncontaining a reducing material are injected into a uniformly appliedmagnetic field from nozzles and immediately mixed as they impinge on oneanother in the form of sprays to cause a reaction between them. However,none of the prior art teaches a method for preparing uniform,agglomerate free submicron/nanosize ceramic powders by incorporating aprecursor solution within a polymeric foam.

SUMMARY OF THE INVENTION

It is an object of this invention to produce submicron/nanosize ceramicpowders.

It is another object of this invention to produce submicron/nanosizeceramic powders without introducing impurities into the powders.

It is another object of this invention to provide a process forpreparing submicron/nanosize ceramic powders which does not requiregrinding of the powder after calcination.

It is another object of this invention to provide a process forpreparing submicron/nanosize ceramic powders in which the conditions forpreparation are not critical to the resulting ceramic powders.

It is yet another object of this invention to provide a process forpreparing submicron/nanosize ceramic/metal powders which is essentiallyindependent of the chemical composition of the ceramic/metal powders tobe synthesized and which is particularly well suited for synthesizingceramic oxides.

It is yet another object of this invention to provide a process forpreparing submicron/nanosize ceramic/metal powders which can be sinteredat temperatures typically a few hundred degrees centigrade lower thanpowders prepared in accordance with the teachings of the prior art.

It is yet another object of this invention to produce ceramic powderswhich are free of agglomerates and uniform in size.

It is another object of this invention to provide a process forpreparing submicron/nanosize ceramic powders in which dopants are easilyadded and stoichiometry is easily controlled.

It is yet another object of this invention to produce ceramic powderswhich are chemically uniform.

These and other objects are achieved in accordance with this inventionin a process for producing ceramic powders in which a polymeric foamcomprising metal cations homogeneously incorporated within a foam cellstructure of the polymeric foam is calcined in an atmosphere conduciveto the removal of the polymeric foam at the lowest temperature and leastamount of time required for complete removal of all organics and theformation of the desired crystal phase, preferably, at a calcinationtemperature of about 400° C. to about 1200° C. for about hour to about 8hours, producing an oxide powder or a metal powder. If metal powders aredesired from the oxide powders produced in accordance with this process,the oxide powders can be reduced to form the desired metal powders. Inaddition, metal powders produced in accordance with this process whichhave a tendency to oxidize upon cooling may be cooled in a reducingatmosphere to prevent oxidation. The powders produced in accordance withthis process are high purity, uniform, agglomerate-free, nanometer size,multicomponent ceramic/metal powders. Powders produced in accordancewith this process range in size from about 5 to about 300 nanometers.

In accordance with one embodiment of the process of this invention,metal cation salts of the desired ceramic/metal composition aredissolved in a solvent system, such as water, acetone, alcohol or othersolvent selected according to its ability to dissolve the metal saltsand for its compatibility with the chemicals used to produce thepolymeric foam. The metal cation solution is then homogeneously mixedwith the organic precursors used to produce the polymeric foam. In apreferred embodiment of this invention, the polymeric foam ispolyurethane foam and the metal cation solution is homogeneously mixedwith the hydroxyl containing component of the components for producingthe polyurethane foam, at a ratio determined from the polymerizationreaction. The resulting chemical solution is then mixed with the otherchemical component needed to produce the polyurethane foam at apredetermined ratio. The chemical mixture at this stage starts to foamalmost immediately at room temperature. Hardening occurs during andafter the foaming is completed producing metal cations homogeneouslyincorporated within the vastly expanded, low density polyurethane foamcell structure. The hardened foams are then heated above thedecomposition temperatures of both the foam and the metal cation saltsto form oxide powders of high purity, uniform, nonagglomerated,nanometer size particles.

Suitable metal cation salts in accordance with this invention are saltswhich are soluble in a solvent which is compatible with the chemicalsused to produce the polymeric foam, such as, chlorides, carbonates,hydroxides, isopropoxides, nitrates, acetates, oxalates, ephoxides andmixtures thereof. To be compatible with the chemicals used to producethe polymeric foam, the solvent must not interfere with or destroy thefoaming process and/or the polymerization process. It is also preferredthat the solvent be completely miscible with one or the other of thechemicals used to produce the polymeric foam.

In accordance with another embodiment of the process of this invention,a polymeric foam is impregnated with a polymeric precursor solution. Theimpregnated polymeric foam is heated at a temperature between about 80°C. to about 150° C. to remove the solvent and the solvent-freeimpregnated polymeric foam is subsequently calcined at a temperature ofabout 400° C. to about 1200° C. for about 1/2 to about 8 hours. Thepolymeric precursor solution in accordance with this embodiment of thisinvention comprises an alpha-hydrocarboxylic acid, a polyhydroxyalcoholand at least one metal cation salt. The alpha-hydrocarboxylic acid andthe polyhydroxyalcohol form a polymeric resin, the type and amount ofwhich depends on the metal cation salt selected. A solvent, preferablywater, is used to dissolve the metal cation salts and polymeric resin toform a solution.

Using a polymeric foam, such as a polyurethane foam, provides supportfor the metal cations or polymeric precursor solution so that the metalcations are evenly dispersed within the foam cell structure. Inaccordance with one embodiment of this invention, the physicalcharacteristics of the polymeric foam skeleton, such as density, poresize, and pore shape, can be used as a supplemental control for theparticle size and morphology of the ceramic powder. Primary control overparticle size and morphology is determined by the amount of solvent usedto dissolve the cation salts and polymeric resin. Powders produced inaccordance with the process of this invention have generally uniformparticle sizes between about 5 nanometers up to about 300 nanometers,have no agglomerates and thus require no grinding after calcination, anddo not contain impurities introduced by the process.

These and other objects and features of this invention will be morereadily understood and appreciated from the following detaileddescription.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the process in accordance with this invention, a polymeric foam, suchas polyurethane foam which is known by the inventors to producefavorable results, comprising metal cations homogeneously incorporatedwithin the foam cell structure of the polymeric foam is calcined in anatmosphere conducive to the removal of the polymeric foam, such as air,at a calcination temperature of about 400° C. to about 1200° C. forabout 1/2 hour to about 8 hours. In accordance with a preferredembodiment of this invention in which the polymeric foam is polyurethanefoam, a metal cation solution having the desired ceramic/metalcomposition is mixed with one of the chemical components used forproducing the polyurethane foam, preferably the hydroxyl containingcomponent. The process of urethane foaming and hardening proceeds in thesame way as in the urethane reaction by adding and mixing the hydroxylcontaining component with an appropriate amount of the isocyanatecontaining component. After foaming and hardening, the metal cations arehomogeneously incorporated within the vastly expanded, low densitypolyurethane foam cell structure.

Various metal cation salts can be used, such as chlorides, carbonates,hydroxides, isopropoxides, nitrates, acetates, ephoxides, oxalates, andmixtures thereof. The type of metal cation salts used are selected onthe basis of solubility, preferably having high solubility in a solventsystem which is compatible with the compounds used for producing thepolyurethane foam. Additional considerations for selecting the cationsalts may include low temperature decomposition of the metal salts,environmentally safe composition of the metal salts, and low cost.

Solvent systems which may be used to dissolve the metal salts comprisewater, acetone, and alcohols. The preferred solvent system is selectedon the basis of its ability to dissolve the desired metal salts and forits compatibility, as previously described, with the compounds used toproduce the polyurethane foam. The preferred solvent is water which candissolve a wide variety of metal salts, which is compatible withpolyurethane making compounds, which is cheap, and which isenvironmentally safe.

Polyurethanes are addition polymers formed by the exothermic reaction ofdi- or poly-isocyanates with polyols in accordance with the followinggeneral reaction: ##STR1##

After reaction with the polyols, the next most important reaction ofisocyanates is with water to yield a substituted urea and carbon dioxidewhich provides the principal source of gas for blowing in the forming oflow density foams. This reaction proceeds in accordance with thefollowing general formula: ##STR2##

It is important to note that water in the system in accordance with thisembodiment of the invention is utilized in the polymerization reactionsand, thus, may not have to be removed by an extra drying process.

To promote the chemical reaction and permit better control of the foamstructure, other chemicals, such as surfactants and catalysts, may beadded. If high purity ceramic/metal powders are desired, then additivescontaining only organic substances ar preferred to avoid theintroduction of impurities.

In accordance with another preferred embodiment of this invention,polyurethane foam is impregnated with a polymeric precursor solutioncomprising a polymeric resin and at least one cation salt. A polymericresin known to be effective in accordance with this embodiment of theprocess of this invention is a mixture of citric acid and ethyleneglycol. The polymeric resin is mixed with at least one cation saltdissolved in a solvent, preferably water, to form a polymeric precursorsolution. Impregnation of the polymeric foam can be accomplished in anynumber of ways, including simply soaking the polymeric foam in thepolymeric precursor solution. The impregnated polymeric foam is heatedat a temperature between about 80° C. to about 150° C.in a desiredatmosphere, preferably air, to drive off the solvent and form dry foam.Following drying, the foam is calcined in a furnace at temperaturesbetween about 400° C. to about 1200° C. for a period of about 1/2 toabout 8 hours, depending on the nature of the ceramic powder and ceramicprecursor under preparation and on the particle size desired, in anatmosphere that allows a complete removal of the polymer anddecomposition of cation salts. A typical heating rate in accordance withone embodiment of the process of this invention is 4° C. per minute in aflow of air. Ceramic powders obtained after calcination are typicallywell crystallized, extremely fine and agglomerate-free, requiring nofurther grinding before being processed and sintered to the desiredform.

EXAMPLE I

This example describes a process for preparing a highly air sinterableLa₀.78 Sr₀.22 CrO_(3-y) (LSC) powder having nanometer size particleswhich can be used to produce interconnects for a solid oxide fuel cell.High purity nitrates, La(NO₃)₃.6H₂ O, Sr(NO₃)₂ and Cr(NO₃)₃.9H₂ O wereused as the starting materials. Appropriate amounts totalling 50 gramsof the nitrates were weighed and dissolved in distilled water,approximately 30 grams, in a 600 milliliter glass beaker. The metalsolution was mixed with 200 milliliters of a mixture of polyols, organicsurfactants and catalysts for approximately five minutes. 200milliliters of a polymeric isocyanate were then added to the chemicalsolution and mixed for one minute. The resulting chemical was quicklytransferred to two 2 L sized porcelain dishes. Foaming occurred almostimmediately and the foam expanded to its maximum volume afterapproximately one-half hour. Hardening was completed in about 40minutes. By heating at approximately 50° C., the time required forfoaming and hardening can be reduced.

The hardened foam was then fired at 750° C., in air, using a heatingrate of approximately 5° C. per minute and a soaking time of five hours.After the firing, the LSC powder obtained was found to be single phaseby x-ray powder diffraction. High resolution SEM showed that the LSCpowder had a typical particle size of about 20 nanometers. The powderwas essentially nonagglomerated and uniform in size.

Subsequently, the LSC powder was compacted into a pellet atapproximately 130 MPa and sintered in air at 1400° C. for approximatelytwo hours to produce a sample having 96 percent of theoretical density.This is compared to LSC powder prepared using known powder processingmethods which produce powders having a particle size of about 1 micron.As a consequence of the larger particle sizes, the powders preparedusing the known methods require a sintering temperature of about 1700°C. under reducing atmosphere conditions to obtain a similar density.

EXAMPLE II

This example demonstrates a process for preparing nanometer size 8 molepercent Y₂ O₃ stabilized ZrO₂ (ysz), which is used in structuralceramics, sensors, and electrolytes in solid oxide fuel cells.

The starting materials used were ZrO (NO₃)₃.xH₂ O and Y(NO₃)₃.xH₂ O.About 50 grams of the nitrates, in a stoichiometric ratio required toobtain Zr₀.84 Y₀.16 O₁.92 was dissolved in approximately 40 gram ofdistilled water. The nitrate solution was mixed in a 600 milliliterglass beaker with a mixture of polyols, organic surfactants andcatalysts for approximately five minutes. Polymeric isocyanates wereadded to the mixture and mixed for approximately one minute and theresulting chemical mixture was transferred to two 2 L size porcelaindishes where foaming rapidly occurred. The foam hardened after aboutthirty minutes and was fired at 750° C., in air, for five hours. Thepowder thus obtained exhibited a single cubic phase of ysz as shown byx-ray powder diffraction. BET study showed that the ysz powder had asurface area of 96 meters squared per gram. SEM studies showed that thepowder had a particle size of about 20 nanometers.

The powder was subsequently die pressed at about 130 MPa and sintered at1250° C. for one minute. A density of about 90 percent of thetheoretical value was obtained. When sintered at 1350° C., ceramicsclose to the theoretical density, approximately 96 percent oftheoretical density, were obtained.

EXAMPLE III

In a modification to the procedure described in Example II, the solventsystem used contained ethanol and acetone in a one to one ratio. Allpreparative procedures were similar to the procedure described inExample II above. A BET surface area of 104 square meters per gram wasmeasured and SEM micrographs showed particle sizes of about 20nanometers.

EXAMPLE IV

This example demonstrates a process for preparing nanometer size Al₂ O₃which is widely used as a structural ceramics.

The starting material used was Al(NO₃)₃.H₂ O. About 50 grams of thenitrate was dissolved in approximately 40 grams of distilled water. Thenitrate solution was mixed in a 600 milliliter glass beaker with amixture (about 150 grams) of polyols, organic surfactants and catalystsfor approximately five minutes. Polymeric isocyanates (about 150 grams)were added to the mixture and mixed for approximately one minute and theresulting chemical mixture was transferred to two 2 L size porcelaindishes where foaming rapidly occurred. The foam hardened after about 30minutes and was fired at 750° C., in air, for 3 hours. The powder thusobtained has a BET surface area of about 118 m² /grams. This correspondsto an average particle diameter of about 13 nm.

EXAMPLE V

This example demonstrates a process for preparing uniform, submicronLiFeO₂ powder as electrode materials for use in a molten carbonate fuelcell.

The starting materials used were LiNO₃ and FE(NO₃)₃.9H₂ O. About 50grams of the nitrates were dissolved in approximately 35 grams ofdistilled water. The nitrate solution was mixed in a 600 milliliterglass beaker with a mixture (about 150 grams) of polyols, organicsurfactants and catalysts for approximately five minutes. Polymericisocyanates (about 150 grams) were added to the mixture and mixed forapproximately one minute and the resulting chemical mixture wastransferred to two 2 L size porcelain dishes where foaming rapidlyoccurred. The foam hardened after about 30 minutes and was fired at 750°C., in air, for 3 hours. The powder thus obtained has a BET surface areaof about 3.8 m² /grams. This corresponds to an average particle diameterof about 0.36 micron. An average particle size of about 0.3 to 0.4micron was observed using SEM.

EXAMPLE VI

This example describes a process for preparing a submicron,nonagglomerated and highly air sinterable La₀.78 Sr₀.22 CrO_(3-y) powderwhich can be used to produce interconnects for solid oxide fuel cells.An appropriate amount of high purity nitrates La(NO₃)₃.H₂ O, Sr(NO₃)₂and Cr(NO₃)₃.9H₂ O were weighed and dissolved in distilled water. Thenitrates to water ratio was 1:1 by weight. Citric acid was alsodissolved in a small amount of water, mixed with ethylene glycol andfinally added to the nitrate solution to obtain a polymer precursorsolution of La(Sr)CrO₃. Approximately 30 grams of the polymeric resinhaving a 1:1 mole ratio of citric acid to ethylene glycol were used forevery 0.05 moles of the nitrates. Polyurethane foam was cut intoapproximately 2 inch by approximately 2 inch by approximately 2 inchcubes which were fully soaked with the precursor solution. Afterallowing excess precursor solution to drip off, the foam cubes wereplaced in an oven and dried at 100° C. for twelve hours. Subsequently,the foam cubes containing the dried precursor were calcined for twohours in a flow of air at 750° C. with a heating rate of 4° C. perminute. The resulting powder has an average particle size of about 0.1microns. As observed under SEM, X-ray diffraction shows a single phaseof LSC.

Sample pellets of the resulting powder, 12.7 millimeters in diameter and2 millimeters thick, were obtained by die pressing at about 130 MPa. Agreen density of about 50 percent of the theoretical was obtained.Sintering was carried out at various temperatures from 1400° to 1600° C.in air for six hours with a heating rate of 20° C. per minute. The finaldensities of the samples were measured using the water immersion methodwith the following results:

                  TABLE                                                           ______________________________________                                        Sintering Temperature                                                                          Relative Density                                             ______________________________________                                        1600° C.  96%                                                          1500° C.  94%                                                          1400° C.  92%                                                          ______________________________________                                    

As shown, the lanthanum strontium chromate powder prepared using thisembodiment of the process of this invention can be sintered to a densityof about 92 percent at 1400° C. in air. In contrast thereto, a lanthanumstrontium chromate powder prepared from the same polymeric precursorsolution without a foam skeleton support during drying and calcinationcan only be sintered to less than 80 percent density at 1600° C.

While in the foregoing specification this invention has been describedin relation to certain preferred embodiments thereof and many detailshave been set forth for purpose of illustration, it will be apparent tothose skilled in the art that the invention is susceptible to additionalembodiments and that certain of the details described herein can bevaried considerably without departing from the basic principles of theinvention.

We claim:
 1. A process for producing at least one of ceramic powders andmetal powders comprising:calcining a polymeric foam comprising metalcations homogeneously incorporated within a foam cell structure of saidpolymeric foam at a calcination temperature for a time required forcomplete removal of all organics and formation of a crystal phase,producing at least one of an oxide powder and a metal powder, saidpolymeric foam produced by mixing an organic precursor comprising amixture of an isocyanate and a hydroxyl-containing compound with atleast one metal cation salt, each of said metal cation salts beingsoluble in a solvent which is miscible in said organic precursor,producing a foam mixture, and hardening said foam mixture.
 2. A processin accordance with claim 1, wherein said calcination temperature isbetween about 400° C. and about 1200° C. and said time is between about1/2 hour and about 8 hours.
 3. A process in accordance with claim 1,wherein said metal powder is cooled to about room temperature in areducing atmosphere.
 4. A process in accordance with claim 1, whereinsaid oxide powder is reduced, forming a metal powder.
 5. A process inaccordance with claim 1, wherein said polymeric foam is polyurethanefoam.
 6. A process in accordance with claim 1, wherein said metalcations are selected from the group of cations consisting of lanthanum,strontium, chromium, zirconium, yttrium, aluminum, lithium, iron, andmixtures thereof.
 7. A process in accordance with claim 1, wherein saidsolvent is water.